Prostate cancer is the most common non-skin cancer and the second leading cause of cancer death among American men. Transrectal ultrasound guided needle biopsy is the most frequently used method for diagnosing prostate cancer due to its real-time nature, low cost, and simplicity. However, the use of ultrasound to detect prostate cancer is limited by its relatively poor image quality and low sensitivity, specifically for prostate cancers. It is difficult to use ultrasound for targeted biopsy guidance because most cancers are not visible under ultrasound. Magnetic resonance (MR) imaging is superior for visualizing the zonal anatomy of the prostate and suspicious focal lesions for prostate cancer. However, MR imaging is costly and the magnetic environment makes interventional procedures more complex, thus, making MRI imaging unsuitable for routine biopsy guidance.
Since MR and ultrasound imaging compliment each other, it is desirable when imaging the prostate to fuse preoperative MR images with real time ultrasound images and take advantage of the superior visualization of MR images in transrectal ultrasound guided biopsy. To do this, the ultrasound transducer is tracked by an exterior localizer that assigns a global coordinate system to the ultrasound images. The registration between the MRI image and the localizer may be carried out using fiducial markers either on the patient or embedded in the patient before the surgical intervention. After both MR and ultrasound are registered to the localizer, the 3D MR image can be reformatted and overlaid in the real-time 2D ultrasound image.
Performing an adequate registration can be difficult. A second problem arises if the transformation between the MR and ultrasound images is determined before the surgical intervention. The system is most accurate when the target organ is static during the procedure. Unfortunately, the prostate moves considerably during biopsy and other types of interventions. The patient often moves involuntarily due to pain or pressure related to insertion of needles or other instruments. Second, a transrectal ultrasound probe can itself cause distortion of the contour of the prostate during the procedure. Finally, respiratory motion of the patient may cause shifting of the prostate, especially when the patient is in a prone position. Since the prostate is a very small organ, the motion can easily result in a loss of accuracy in an MR/transrectal ultrasound fusion display, leading to inaccurate needle insertions when using the fused display for targeted biopsies. Sometimes, the MR and ultrasound images can be completely disconnected from each other, making the MR image useless for surgical navigation.
Accordingly, there is a need for instrumentation, systems, and method for obtaining and maintaining registration between preoperative images and intraoperative images, and determining the location of instrumentation on any of these images in patient space.